THE 1>RL\CIPLES OF MYODYNAMICS. IO7 



long- axis of the tibia, and the Hne d j c is in the 

 conjugate diameter of the tarsus : These Hnes 

 meet at the angle b j d The line b j represents 

 the retentive pressure of the tibio-fibular arch 

 on the astragalus, and the line b d represents 

 the inward displacing force due to the obliquity 

 of the tarsus : because the right angled triangles 

 d j b and a j c are similar — D : R : : D, : R. I 

 have treated a dislocation of the Toot, compli- 

 cated with Potts' fracture, in which the sole - 

 cause was the displacing component marked D in 

 the f'^ig., a heavy weight having fallen on the top 

 of the knee, when the foot rested on the ground 

 and the leg was perpendicular to the foot : The 

 force applied in this case must have been very 

 great. 



102. (B.) Fig. 23 represents an inside view 

 of the bones of the foot with the lower end of 

 the tibia : d A B is the plantar arch, and P-F 

 is the plantar fascia. The astragalus A is the 

 key-bone of the arch, having the weight nearer 

 the posterior than the anterior pillar. When 

 the heel is raised the weight of the body rests 

 on the anterior pillar, and when the toes are 



